"The persistence of rainfall patterns during HAPEX-Sahel has provided the first observational evidence that the land surface influences rainfall in the region."

• Sahelian drought
• Observations from a land surface experiment
• Feedback mechanisms
• Outlook
• References
• Author information
• Additional web resources

Sahelian drought

Thumbnail link to graph of rainfall in Sahel

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Lying between the Sahara desert to the north and moist tropical savanna to the south, the African Sahel is a region which suffers from very variable annual rainfall. In recent years, however, there has been a prolonged drought, with annual rainfall only exceeding the long-term average in a handful of years since 1968. Because of this, the region has become a focus of much scientific effort to try to understand the underlying causes of the drought. An important aspect of this research has examined the role of feedbacks between the land surface and the atmosphere. Charney (1975) originally hypothesized that a reduction in vegetation cover over the Sahel could disrupt the regional atmospheric circulation, reducing rainfall and contributing to the persistence of arid conditions.

Since Charney published his hypothesis, there has been a great deal of argument about the efficiency of this feedback mechanism, and its actual role in the observed drought. Studies based on computer simulations of the atmosphere and land surface have been used extensively to explore the potential for the mechanism. For example, Xue (1997) has argued that land degradation, associated with processes such as the clearing of natural open woodland for agriculture, can result in climatic changes as dramatic as those observed in recent times. Whilst degradation at the local scale has been well-documented (Mabbutt and Floret 1980), it is rather more difficult to quantify the changes in land use over the entire region. The idea that severe degradation has occurred throughout the Sahel and caused the drought has been criticized by some (e.g. Nicholson et al. 1998). A complicating factor in understanding the influence of degradation on drought is that the climate system is itself highly variable. A number of studies (e.g. Folland et al. 1986, Rowell et al. 1995) show that rainfall in the Sahel is closely related to natural variability in the global climate system. Conditions favorable for wet or dry years are provided by sea surface temperature patterns in the Atlantic and Pacific Oceans. Such "natural" climatic variability has a pronounced effect on the vegetation cover (Prince et al. 1998). In a single wet year, vegetation can thrive and the surface appears much greener than usual when viewed from satellite. Even after several dry years, vegetation can recover relatively quickly once adequate rainfall returns. From this perspective, it is difficult to determine whether changes in vegetation are a cause or a result of the drought.

Observations from a land surface experiment

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To isolate the impact of changes in vegetation on the regional climate, we must therefore rely on computer climate models. In an attempt to improve the way that these models represent the complex interactions between the land surface and atmosphere, and to observe some of the feedback processes directly, the Hydrological-Atmospheric Field Experiment in the Sahel (HAPEX-Sahel; Goutorbe et al. 1994) was conducted during 1992. The study area covered a 1 degree by 1 degree square (roughly 110 x 110 km) around Niamey, Niger, in a region which typically receives 560 mm rainfall per year. Roughly 90% of the annual rainfall occurs between June and September, when the West African monsoon brings moist air from the Gulf of Guinea over the continent. Prior to the onset of the wet season, the soil is extremely dry, grassy cover has long since died, and only deeply rooted woody species can transpire. However, the vegetation develops rapidly once rainfall has moistened the root zone, and the observations show how this signals a change in the effect of the land surface on the atmosphere. The growing vegetation and moister soil are able to absorb more solar energy than the previously arid and sparsely vegetated surface. In addition moisture is now available for evaporation, both at the soil surface and in the root zone. These changes affect the temperature and humidity of the lower atmosphere, making rainfall more likely. This is an example of a positive feedback where the land surface affects the climate.

Thumbnail link to map of rainfall differences in study area

The HAPEX-Sahel experiment also provided a very detailed picture of rainfall in the region for the first time. Based on measurements at over 100 rain gauges (typically every 12 km), the data showed that very large differences in annual rainfall can occur even over small distances. The spatial patterns of precipitation over the study area during 1992 were quite complex. The long-term climatology of the region presents a decrease in rainfall of roughly 100 mm for every 1 degree north traveled. It is rather difficult to see this gradient in a single year, such is the variability of rainfall on spatial scales as small as 10 km. Only when one takes several years of data together does the decrease in rainfall with latitude become apparent. The largest rainfall gradient during 1992 occurred in the south-west of the study area, where one gauge received 295 mm more rain than its neighbor just 9 km away. What was extraordinary about this extreme variability was that during the first half of 1992, and in other years, these neighboring gauges received very similar amounts of rainfall. The gradient in 1992 developed as a result of a series of storms passing over in the final seven weeks of the wet season, each exhibiting the same spatial pattern (Taylor et al. 1997; Taylor and Lebel 1998).

Thumbnail link to maps of rainfall for 2 events, July 1992

Further analysis of the rainfall data revealed that this persistence of rainfall patterns from storm to storm occurred elsewhere in the study area. This can be demonstrated by analysis of rainfall for two consecutive events, on 20 and 22 July 1992. On 20 July, some short-lived local storms developed during the afternoon, producing rain in the center of the study area. The event on 22 July, on the other hand, was a typical tropical squall line. Squall lines are long-lived storms which tend to come from the east, producing intense rainfall in a swath which can be several hundreds of kilometers across. Whilst all gauges in the domain received at least 5 mm rain on 22 July, in some areas precipitation exceeded 55 mm. What is striking about the two plots is that over the central area, those gauges which had rain on 20 July were favored with much higher rainfall than their neighbors in the event on 22 July. Analysis of the statistics of these events confirms that, on average, when a squall line passes over, rainfall is likely to be heavier in an area where rain has fallen in the previous couple of days. This unexpected behavior points strongly towards a positive land surface feedback operating locally.

Feedback mechanisms

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To understand how a feedback might operate, it is useful to consider what happens to the surface after rainfall. In the sandy soils found throughout much of the Sahel, the surface remains wet for a day or two after rain, as the water drains slowly down into the soil. During this period, the soil surface is cooler and less reflective, and this provides more energy for heating and moistening the atmosphere. In addition, direct evaporation from the wet soil is strong. All else being equal, the net result is that in areas which have been rained on, the lower atmosphere will be slightly moister than in dry areas. If a wet area is only small, one would expect the extra humidity from the soil to be rapidly mixed with drier air by the wind. Observations from HAPEX-Sahel, however, show that at the scale of 10 km a "memory" of recent storms does remain in the atmosphere through differences in humidity. Furthermore, the observations show that when rainfall patterns persist over many storms, vegetation may develop more strongly in the wet areas. This ensures that high rates of evaporation can be maintained within the wet areas, even during extended rain-free periods when the topsoil has dried out.

The final part of the positive feedback loop requires that the extra humidity above the wet surface can increase rainfall. It is hypothesized that the humidity gradients play an important role in modulating the dynamics of squall lines locally when they pass over. Squall lines derive their energy from moisture in the lowest layers of the atmosphere, and from a simple energetics perspective, more moisture being fed into the storm at low levels is likely to result in more vigorous convection, and hence rainfall. This hypothesis is currently being tested with a numerical model of a squall line, and initial results suggest that rainfall is indeed heavier over wet soils.

The variability in seasonal rainfall can have dramatic effects on local communities. For example during 1992, generally low rainfall in the north of the study area was exacerbated by a persistent local minimum around several villages. As a result, the millet crop failed and all the adult males within a 10-15 km radius traveled hundreds of miles to the coast in order to make enough money to survive the dry season.

Outlook

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The persistence of rainfall patterns during HAPEX-Sahel has provided the first observational evidence that the land surface influences rainfall in the region. The lack of other evidence is due not so much to the weakness of feedbacks between the land and atmosphere as to the problem of collecting enough observations to isolate the influence of the land surface from other factors. Many of the mechanisms highlighted above operate at a larger spatial scale as well. For example, during a relatively dry year the surface will reflect more solar energy than usual, and both bare soil evaporation and transpiration will be weak. These effects may influence key factors in determining annual rainfall, such as the initiation and life times of squall lines, and at the regional scale, the strength of the monsoon winds. It is also unclear how much recent deforestation, both in the Sahel and further south towards the Gulf of Guinea, have affected the climate of the area. What is known, however, is that the geological record shows periods of conditions both wetter and drier than today (Nicholson 1989). A recent study (Claussen et al. 1999) has shed some light on the nature of these transitions. Their modeling study focussed on the transition from a "green" Sahara to its current arid state during the mid-Holocene era (9000 to 6000 years ago). They suggest that whilst the primary forcing was subtle changes in the earth's orbit, vegetation feedbacks played a pivotal role in a rather abrupt climatic shift.

From the many studies produced since the onset of the drought, a picture emerges of a rather precarious climate in the Sahel. We have seen that natural variability in the climate system can cause enormous differences in rainfall even between neighboring villages. At the larger scale, severe droughts can affect the entire region over not just years, but decades, and even centuries. At all of these time scales, land surface feedbacks play an important role, and large-scale changes in land use may only increase the climatic vulnerability of the region. For example, the conversion of savanna and natural forest to cropland is expected to accelerate in the coming years (Lambin 2001) under rising population pressures. Currently, there is no general consensus on the likely overall impacts of global climate change on the Sahel. Some climate centers predict increases in Sahel rainfall due to higher levels of carbon dioxide, and others suggest lower levels of rainfall. However, at least one thing seems clear: given the natural variability of rainfall in the Sahel, coupled with both unprecedented changes in regional land use and the uncertainties associated with global changes in greenhouse gases, climate stability in the Sahel is unlikely.

References

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Charney, J.G. 1975. Dynamics of deserts and drought in the Sahel. Quarterly Journal of the Royal Meteorological Society 101:193-202.

Claussen, M., C. Kubatzki, V. Brovkin, A. Ganopolski, P. Hoelzmann, and H.J. Pachur. 1999. Simulation of an abrupt change in Saharan vegetation in the mid-Holocene. Geophysical Research Letters 26:2037-2040.

Folland, C.K., T.N. Palmer, and D.E. Parker. 1986. Sahel rainfall and worldwide sea temperatures, 1901-85. Nature 320:602-607.

Goutorbe, J.P., T. Lebel, A. Tinga, P. Bessemoulin, J. Brouwer, A.J. Dolman, E.T. Engman, J.H.C. Gash, M. Hoepffner, P. Kabat, Y.H. Kerr, B. Monteny, S. Prince, F. Saïd, P.J. Sellers, and J.S. Wallace. 1994. HAPEX-Sahel - a large-scale study of land-atmosphere interactions in the semi-arid tropics. Annals of Geophysics 12:53-64.

Mabbutt, J.A. and C. Floret, eds. 1980. Case studies on desertification. Paris: UNESCO.

Nicholson, S.E. 1989 Long-term changes in African rainfall. Weather 44: 47-56. Nicholson, S.E., C.J. Tucker, and M.B. Ba. 1998. Desertification, drought, and surface vegetation: An example from the West African Sahel. Bulletin of the American Meteorological Society 79:815-829.

Prince, S.D., E.B. De Colstoun, and L.L. Kravitz. 1998. Evidence from rain-use efficiencies does not indicate extensive Sahelian desertification. Global Change Biology 4:359-374.

Rowell, D.P., C.K. Folland, K. Maskell, and M.N. Ward. 1995. Variability of summer rainfall over tropical North-Africa (1906-92) - Observations and modeling, Quarterly Journal of the Royal Meteorological Society 121:669-704.

Lambin, E.F. 2001. Personal communication with the author.

Taylor, C.M., F. Saïd, and T. Lebel. 1997. Interactions between the land surface and mesoscale rainfall variability during HAPEX-Sahel. Monthly Weather Review 125:2211-2227.

Taylor, C.M. and T. Lebel. 1998. Observational evidence of persistent convective-scale rainfall patterns. Monthly Weather Review 126:1597-1607.

Xue, Y. 1997. Biosphere feedback on regional climate in tropical north Africa. Quarterly Journal of the Royal Meteorological Society 123:1483-1515.